Alzheimer's disease (AD) is a leading cause of dementia and affects over 4.5 millions Americans at a total cost of at least $100 billion per year. Deregulation of the protein, tau, and altered processing of the amyloid precursor protein (APR) are key events in AD pathogenesis. Clinical studies suggest that dietary folate deficiency is associated with elevated plasma levels of homocysteine (Hey), a risk factor for AD and age- related cognitive decline. There is increasing evidence that protein phosphatase 2A (PP2A), a family of major brain enzymes, is involved in AD pathogenesis. PP2A catalytic subunit can be methylated by a specific methyltransferase (PPMT), and demethylated by a dedicated methylesterase (PME-1), whose properties and regulation in neurons have not yet been explored. Methylation modulates PP2A functional specificity. Notably, PPMT activity is inhibited in vitro by the Hey precursor, S-adenosyl-L-homocysteine (SAH), and SAH levels are elevated in AD brain. Significantly, our preliminary data indicate that: 1) Neuronal PPMT and PP2A methylation are downregulated in AD; 2) demethylated PP2A accumulates in the brain of mice fed a low-folate diet; and 3) reduced PP2A methylation correlates with alterations in tau and APP. The goal of our research proposal is to further test the hypothesis that PPMT is a critical intermediate between folate-dependent Hey metabolic pathways that are a risk factor for AD, PP2A, and the regulation of tau and APP. As such, folate deficiency may induce downregulation of PP2A methylation and contribute to AD pathogenesis. In Aim 1, silencing strategies and expression of wild-type and mutant proteins in cultured cells will be utilized to investigate the effects of deregulating PPMT and PME-1 on the regulation of PP2A, tau, APP and microtubules. In Aim 2, the hypothesis that chronic dietary folate deficiency inhibits PP2A methylation and promotes tau phosphorylation and amyloidogenesis will be further tested in wild-type and transgenic AD mouse models. In Aim 3, Western blot and immunohistochemical analyses will be performed to compare the distribution of PPMT, PME-1 and demethylated PP2A in human brain regions during normal aging, and assess whether it becomes altered in AD and other tauopathies. If our model is validated, dietary folate supplementation and PPMT may be novel therapeutic targets for strategies aimed at slowing down the disease process in AD, and possibly other neurodegenerative dementias. [unreadable] [unreadable]